The present invention relates generally to beaded rear projection screens, and more specifically to rear projections screens that incorporate beads of different refractive indices.
Rear projection screen displays based on glass microspheres (or beads) embedded in an opaque matrix have been growing in popularity for various uses, such as in large format televisions. A rear projection screen is a sheet-like optical device having a relatively thin viewing layer that is placed at an image surface of an optical projection apparatus. Such a screen makes visible a real image focused by a projection apparatus onto the image surface. The viewing layer is typically planar corresponding to the image surfaces produced by a projection apparatus. Other shapes are possible if the image surface of the projection apparatus is not planar. The screen is intended to act as a filter to attenuate, block, or diffuse light which is not part of the projected image, and to transmit from its rear side to its front side that light which is part of the projected image. In this way it enables the viewer to see the projected image when looking at the front side of the screen.
In a typical construction, the transparent microspheres are embedded in an opaque binder material with the rear portion of the microspheres exposed to light from the projection apparatus. The front portions of the microspheres may extend partly through the opaque binder material to contact the screen substrate. The contact areas form optical apertures between each microsphere and the substrate. The area surrounding each optical aperture is opaque, and preferably black, due to the opaque binder material in the microsphere interstices. As a result, ambient light incident in these areas can be absorbed.
In some circumstances, it might be desirable to have the capability to provide beaded rear projection screens having a particular gain or viewing angle for one application and to provide beaded rear projection screens having different gains or viewing angles for one or more other applications, all while using the same beads. For example, a screen manufacturer might have a limited selection of bead indices due to available inventories, or for other reasons might prefer to use only certain index beads, while at the same time having different customers that desire screens having different gains, viewing angles, or other performance characteristics. The present invention provides a solution. In the present invention, a rear projection screen construction can be determined in which two or more pre-determined bead types with different refractive indices can be mixed in various ratios to tune the gain, viewing angle, or other performance characteristics within a desirable range.
The present invention amounts to more than a realization that gain and other screen properties can be tuned by mixing different index beads. In the present invention, it is also recognized that the screen construction can be modified or selected so that the chosen beads result in a high performance screen regardless of the mixing ratio. The present invention further provides a method of selecting bead indices for a given screen construction to achieve highly predictable tuning of screen performance while maintaining relatively high transmission. As such, the present invention provides rear projection screens incorporating two or more different index beads, and additionally provides the ability to tune gain and viewing angle of these screens over a wide range without significantly sacrificing throughput.
In one aspect, the present invention provides a method of making a beaded rear projection screen. The method includes determining a model screen construction that includes a substrate transmissive to visible light, a visible light absorptive material disposed on the substrate, a layer of microbeads partially embedded in the light absorptive material to leave a side of the microbeads exposed, and an optional overcoat disposed over the exposed side of the microbeads. The method further includes calculating throughput for the model screen construction as a function of microbead refractive index so that a theoretical maximum throughput can be determined. The method also includes determining a range of microbead refractive indices such that any refractive index in the range when used as the refractive index of the microbeads in the model screen construction would result in a calculated screen throughput of about 90% or more of the theoretical maximum throughput. Finally, the method includes choosing a first microbead type having a first refractive index within the determined range, choosing a second microbead type having a second refractive index within the determined range, the second refractive index being different from the first refractive index, and mixing and dispersing a ratio of the first microbeads and the second microbeads to form a substantially uniformly dispersed layer of microbeads for a beaded rear projection screen that has a construction that functionally matches the model screen construction. By xe2x80x9cfunctionally matchesxe2x80x9d it is meant that the beaded projection screen that is made has essentially the same construction as the model screen (except for including multiple bead types rather than a single bead type), and that minor variations in layer thicknesses, refractive indices, and other properties may exist.
In another aspect, the present invention provides a method for making a beaded rear projection screen that includes the steps of providing a first plurality of microbeads having a first index of refraction, providing a second plurality of microbeads having a second index of refraction, and determining a model screen construction. The model screen construction includes a substrate transmissive to visible light, a visible light absorptive material disposed on the substrate, a layer of microbeads partially embedded in the light absorptive material to leave a side of the microbeads exposed, and an optional overcoat disposed over the exposed side of the microbeads, wherein the model screen construction has a theoretical maximum throughput for a given microbead refractive index, and wherein the model screen construction is determined such that when microbeads having the first index and microbeads having the second index are present as the layer of microbeads, the resulting screen has a theoretical throughput that is about 90% or more of the maximum theoretical throughput. The screen can then be made by mixing and dispersing a ratio of the first type of microbeads and the second type of microbeads to form a uniform layer of microbeads for a beaded rear projection screen that has a construction that is substantially the same as the model screen construction.
In another aspect, the present invention provides a rear projection screen that includes a beaded screen construction comprising a substrate transmissive to visible light, a visible light absorptive material disposed on the substrate, and a layer of microbeads embedded in the light absorptive material to leave a portion of the microbead layer exposed, wherein the beaded screen construction has a maximum throughput, and nTmax defines a microbead index of refraction at which the maximum throughput is attained, and wherein the layer of microbeads comprises a plurality of first microbeads having a first index of refraction and a plurality of second microbeads having a second index of refraction, the first index of refraction being in a range of nTmaxxc2x10.05, inclusive, and the second index of refraction being less than the first index of refraction.
In yet another aspect, the present invention provides a rear projection screen that includes a beaded screen construction comprising a substrate transmissive to visible light, a visible light absorptive material disposed on the substrate, and a layer of microbeads embedded in the light absorptive material to leave a portion of the microbead layer exposed, wherein the beaded screen construction has a maximum theoretical throughput, nT defines a microbead index of refraction at which the maximum theoretical throughput is attained, and na to nb defines a range of microbead indices that includes nT and all other refractive indices that yield a calculated throughput of about 90% or more of the maximum theoretical throughput, and wherein the layer of microbeads comprises a first plurality of microbeads that have a refractive index of about na and a second plurality of microbeads that have a refractive index of about nb.
In still another aspect, the present invention provides a rear projection screen that includes a substrate transmissive to visible light, a visible light absorptive material disposed on the substrate, and a layer of microbeads partially embedded in the light absorptive material to leave the microbeads exposed to an air interface, the layer of microbeads consisting essentially of a uniform dispersion of about 1.65 refractive index microbeads and about 1.5 refractive index microbeads.
In a further aspect, the present invention provides a rear projection screen that includes a substrate transmissive to visible light, a visible light absorptive material disposed on the substrate, and a layer of microbeads partially embedded in the light absorptive material to leave the microbeads exposed to an air interface, the layer of microbeads consisting essentially of a uniform dispersion of about 1.65 refractive index microbeads and about 1.5 refractive index microbeads, wherein the rear projection screen has a gain that corresponds to the gain of a theoretical screen having the same construction but with a layer of microbeads that includes only microbeads having an index of refraction about equal to (1.65a+1.5b)/(a+b), where a:b is the ratio of 1.65 index microbeads to 1.5 index microbeads.